Fuel cells, in general, have a multiple of cell units which are layered to form a stack, and each of the cell units has a structure that electrodes serving also as catalyst layers are so disposed as to sandwich an electrolyte layer and a separator is disposed outside each of the electrodes. The separators function as a partition wall of a passage for a gas such as hydrogen gas and oxygen gas or for cooling water, and a gasket made from a rubber is provided therewith for the purpose of sealing against the gas or the cooling water.
The rubber gasket is sandwiched between the separators during assembly of a fuel cell and compressed to a predetermined thickness In that case, a compression ratio of the rubber gasket is typically set to 15 to 60% in view of a variation in separator thickness and a gap between the separators to which the gasket is placed. Though a sealing property is improved when the compression ratio is high, a reaction force (a force of pressing against the separators to resist the compression) of the rubber gasket is increased due to the high compression ratio, and when the compression ratio is too high, a problem of breakage of the separators occurs. The thickness and the width of the rubber gasket may be reduced so as to reduce the reaction force of the rubber gasket; however, in the case of a low compression ratio, such modification leads to an insufficient reaction force which causes a problem of deterioration in sealing property. Rubber gaskets involve such antinomy problems in the relationship between the size and the compression ratio irrespective of its usage.
Various rubber gaskets modified in shape have heretofore been proposed for the purpose of solving the above-described antinomy problems. For example, as shown in a sectional view of FIG. 2, a rubber gasket having a main bead portion 5 obliquely projecting to the inner side from an edge of a substrate 4 whose section has a substantially rectangular shape is known (see, for example, Patent Document 1). The rubber gasket is readily compressed because the main bead portion 5 is easily bent down upon mounting and has a small reaction force, and a sufficient sealing property is ensured by the bent main bead portion 5. However, though the main bead portion 5 functions as described above in the case where the rubber gasket is made from a thermoplastic elastomer having a hardness of 20 to 50 (durometer; type A), a crack is caused in the bent main bead portion 5 due to compression in the case where the rubber gasket is made from a rubber having a higher hardness, thereby causing a problem of a reduced sealing property.
Further, as shown in a sectional view of FIG. 3, a rubber gasket having a bead-like sealing lip 7 which is formed on a lower flat sealing structure 6 of a substantially trapezoidal shape has been proposed (see, for example, Patent Document 2). However, this rubber gasket has a problem that a reaction force is increased too much when the compression ratio is 60% due to a large sectional area of the lower flat sealing structure 6.
Patent Document 1: JP-A-2003-49949
Patent Document 2: JP-A-2003-56704